1. Field of the Invention
The present invention relates to a fixing apparatus used in an image-forming apparatus such as electrostatic copying machines, printers, or facsimiles, utilizing electrophotographic process. More specifically, the present invention relates to a fixing apparatus used for fixing visible images transferred onto a recording medium, and to a fixing method using such a fixing apparatus.
2. Discussion of the Related Art
In conventional fixing methods and fixing apparatus, used in an image-forming apparatus for fixing visible images transferred onto a recording medium, utilizing electrophotographic process, the following methods have been mainly disclosed and practically used:
(a) A pressure-fixing method for fixing a visible image using a high nip pressure exerted by a pair of metal rollers; PA1 (b) A heat roller fixing method for fixing a visible image using a heat roller comprising a hollow metal cylinder having a heat source in the inner portion thereof, the surface of the hollow cylinder being coated with a releasing layer such as TEFLON coating, and a pressure roller comprising a metal shaft and a silicone rubber layer formed in the periphery of the metal shaft; and PA1 (c) A film fixing method for fixing a visible image using a thin film and a linear heating member, in which the pressurizing portion necessary for fixing is intensively heated, thereby shortening the waiting time for the temperature rise in the fixing device. PA1 (1) A fixing apparatus for fixing visible images formed on a recording medium, the fixing apparatus comprising a heat roller having a heating means in the inner portion thereof, and an elastic member forming a nip in contact with the heat roller and capable of exerting a pressure at the nip portion by an elastic stress, wherein the recording medium is fed into the nip portion; and PA1 (2) A fixing method comprising the step of carrying out heat-and-pressure fixing of visible images by feeding a recording medium having the visible images formed thereon through the nip portion using the fixing apparatus described in (1) above.
Further, (d) U.S. Pat. No. 4,689,471 and Japanese Patent Laid-Open Nos. 1-304481 and 2-33175 disclose a heat-and-pressure fixing apparatus comprising a light-weight fixing roller (a heat roller) and a pressure-exerting member of an elongated rectangular shape forming a nip with the fixing roller, wherein fixing is carried out by contacting the fixing roller and a visible image transferred onto the recording medium, and modified apparatus thereof.
Among the above methods, the heat roller fixing method (b) has been known as a typical fixing method. In this method (b), a fixing apparatus comprises a pair of rollers wherein at least one of the rollers is a heat roller having a heat source such as a halogen lamp in the inner portion, and the other roller is a pressure roller coated with a surface layer comprising an elastic layer made of rubbers, etc. The pair of rollers rotates in contact with each other with a suitable nip width by a driving means (see FIG. 2) wherein 2 is a heating means, element 3 a heat roller, element 3a a metal roller, element 3b a rubber layer, element 4 a pressure roller, element 4a a metal shaft, element 4b a rubber layer, element 10 a recording medium, element 11 a visible image, and element Wa a nip width
However, in the pressure-fixing method (a), a roller with a high surface smoothness is required. Also, in order to achieve good fixing ability, a nip pressure of not less than 10 kg/cm has to be exerted, so that a sturdy structure would be necessary. Therefore, the overall apparatus itself becomes large.
Further, in the heat roller fixing method (b), since fixing is carried out using a heat roller having a heat source in the inner portion thereof together with a pressure roller, the degree of miniaturization is limited. Also, since loads are applied on both ends of the shaft, the rigidity of the roller has to be increased, and crossed axes angle has to be set, in order to evenly exert a nip pressure at the nip portion.
Further, in the film fixing method (c), the overall system is complicated, and the cost of the system entailed by the temperature control and the film deflection control undesirably increases about 1.5 times that of the heat roller fixing method.
Further, in the methods (d) disclosed in U.S. Pat. No. 4,689,471 and Japanese Patent Laid-Open No. 1-304481, etc., since pressurizing is carried out by a separate means, the overall system becomes complicated, thereby making the apparatus large. Also, since the nip width is large, offsetting of the toner is likely to take place at the nip portion of the fixing roller.